"Y520 has lower battery life" - No, it has almost no battery life!
The Lenovo Legion typically ships with an IPS panel, which makes it much more useful as a portable device compared with a TN panel. Sadly, my comment about battery life is no joke - it really is 2 - 3 hours of productivity and 1 hour or less of actual gaming. But you get a slightly better CPU and a much faster SSD system disk.
Check the panel details on the Dell. Battery life is likely to be better, but no by much though - it's running pretty much the same CPU/GPU setup - and you'll have to make do with a mechanical HDD (or buy an SSD later). If the Dell comes with an IPS panel, I would actually recommend it over the Legion - battery life can't possibly be worse than the Legion. But if it comes with a TN panel, you're really in a dilemma here:
The Legion offers the superior panel but you have to bring the power cord everywhere you go. The Dell may dispense with a power cord (for a few hours more) but the TN panel will make you regret your decision every time you open the machine.